Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
This circuit is from an August 1984 Radio Electronics magazine. It monitors the state of a RAM...
Analog Devices suggests this circuit in documentation from the 1990s. The circuit is for 75-ohm...
This circuit, which can be built with several CMOS gates, was found in the March 1996 issue of...